Heretofore, diversity receivers of the type described are used to suppress waveform distortions due to fading in transmission paths for providing a good communication environment in microwave digital radio communication systems. One of such diversity receivers is a space diversity receiver for receiving radio signals with a plurality of reception antennas that are spaced at sufficient distances from each other and combining a plurality of minimally correlated received signals from the reception antennas for thereby reducing reception distortions.
There have been proposed various processes of combining a plurality of received signals in such space diversity reception. Particularly, three of those proposed combining processes, i.e., “in-phase combining process”, “minimum amplitude deviation type combining process”, and “maximum level combining process” are mainly used in the art. The “in-phase combining process” combines received signals from a plurality of reception routes such that the received signals are in phase with each other, and is disclosed in Japanese laid-open patent publication No. 8-274695.
The “minimum amplitude deviation type combining process” detects a notch of a combined received signal and combines received signals so as to minimize an in-band amplitude deviation of the combined received signal depending on the detected notch, is disclosed in Japanese laid-open patent publications Nos. 59-230333 and 7-38536.
The “maximum level combining process” combines received signals such that the combined level thereof will be maximum. According to either one of the above three combining processes, at least one of the received signals is rotated in phase by an endless phase shifter for signal combination. The above three combining processes have both advantages and disadvantages, and are often used in combination. In particular, it is the general practice to use the “minimum amplitude deviation type combining process” and the “maximum level combining process” in combination with each other.
A conventional diversity receiver based on a combination of the “minimum amplitude deviation type combining process” and the “maximum level combining process” will be described below. FIG. 1 of the accompanying drawings shows such a conventional diversity receiver. In FIG. 1, received signals #1, #2 are signals that have been received by respective diversity antennas (not shown) and have their frequencies converted into an intermediate frequency range. EPS (Endless Phase Shifter) 1 is supplied with received signal #2 and an output control signal from EPS control circuit 8, and rotates the phase of received signal #2 through unit steps (degrees) in either direction based on the output control signal, and outputs received signal #2 which has been rotated in phase.
Combiner 2 is supplied with the output signal from EPS 1 and received signal #1, combines these signals, and outputs the combined signal. Level detector 4 is supplied with the combined signal from combiner 2, detects the signal level of the combined signal, and outputs the detected signal level. DEM (Demodulator) 6 is supplied with the combined signal from combiner 2, determines and outputs the sign of the combined signal.
Notch detector 7 is supplied with the combined signal from combiner 2, detects and outputs the depth (distortion) of a notch thereof. EPS control circuit 8 is supplied with the output signal from notch detector 7 and the output signal from level detector 4, generates an optimum control signal from the supplied signals, and outputs the control signal to EPS 1.
Operation of the conventional diversity receiver shown in FIG. 1 will be described below. FIG. 2 of the accompanying drawings illustrates how to switch between the combining processes. The horizontal axis of FIG. 2 represents the depth of the notch (=the distortion of the waveform), and the vertical axis the combined signal level. The depth of the notch and the combined signal level are greater in the respective directions indicated by the arrows. In FIG. 2, the “maximum level combining process” is performed, i.e., the combined signal level is maximized (level improved), in region A. Specifically, EPS control circuit 8 monitors only the output signal of level detector 4 and applies an output control signal to EPS 1 to rotate the phase of received signal #2 in order to keep the combined signal level maximum.
In region B shown in FIG. 2, the “minimum amplitude deviation type combining process” is performed, i.e., the in-band deviation of the combined signal is minimized (distortion improved). Specifically, EPS control circuit 8 monitors the output signal of notch detector 7 and applies an output control signal to EPS 1 to rotate the phase of received signal #2 in order to minimize the distortion of the waveform. More specifically, notch detector 7 detects levels at a plurality of frequencies in the spectrum of the received signal to detect the magnitude of a change in the amplitude vs. frequency characteristics in the band of the received signal, and EPS control circuit 8 controls EPS 1 to minimize the in-band amplitude vs. frequency deviation.
If it is judged that the combined signal level is high (higher than “b” in FIG. 2) and the distortion is small (smaller than “a” in FIG. 2), then the level improving process is performed. In the distortion improving process, since the in-band deviation is minimized depending on the magnitude of the notch, as described above, main components of the received signals often tend to be combined at angles close to opposite phases, and the amplitude of the combined signal becomes low in level though it is flat. If the distortion is small, then because a reduction in the signal level poses a greater problem as to an error rate than the distortion, region B is not used, but region A is used in preference for a S/N (signal level to noise level) ratio.
If only the distortion becomes greater (greater than “a” in FIG. 2), then control goes to the distortion improving process. If the distortion is large as in region B and the distortion has a greater effect on the error rate than the level, then the distortion improving process is performed as a sufficient distortion improving capability is expected, but the level is lowered. If the level becomes lower than “b” in FIG. 2 at this time, then control goes to the level improving process because the level reduction poses a serious problem though some distortion may remain unremoved (some distortion can be equalized by DFE (Decision Feedback Equalizer) if DFE is provided at a subsequent stage).
The problem of the conventional arrangement is that the phase control based on the EPS is liable to be unstable if the level improving process is performed while the distortion is small and the level is high. This is because the level for use in the control signal is usually detected by a diode and it is highly difficult to detect slight level changes with the diode. The unstable phase control is responsible for a large reduction in the system stability.